Among various types of backlight modules, a single-side light entry type is now used more and more widely. Since it merely needs to dispose a light source at one side of a light guide plate for the single-side light entry type, the number of printed circuit boards, heat dissipation lumps for source of light, light sources, light driving boards, components, channels, conductive lines, and connectors can be reduced and it is convenient for the assembly.
Referring to FIG. 1, FIG. 1 is a top view of a conventional backlight module of a single-side light entry type. Specifically, the aforesaid conventional backlight module is of a single-long-side light entry type.
The backlight module comprises a light source 11 and a light guide plate 12. The light source 11 has an elongated shape. The light guide plate 12 comprises a long edge 121 and a short edge 122. The light source 11 is disposed corresponding to the long edge 121. The light source 11 and the light guide plate 12 have positioning posts 13 disposed therebetween.
Since the light guide plate 12 is made of a plastic material, the plastic material will be changed or affected by external circumstance. Take a Poly(methyl methacrylate) (i.e., PMMA) for example. Under a moist circumstance, the water absorption of PMMA may reach 2% in weight and the volume of the light guide plate 12 is caused to change. The water absorption rate is a percentage of the weight of absorbed water out of the net weight of material when the material is saturated with water. For example, if the original width of the light guide plate 12 is 395 mm, the light guide plate 12 may be increased for 1.62 mm in width once the water absorption rate reaches 2%. Also, the light guide plate 12 may be caused to change by an excessive temperature under a high-temperature circumstance.
When the light guide plate 12 is caused to change, the change direction is along a gravitational direction A′ of the light guide plate 12 due to the action of gravity of the light guide plate 12 itself. In order to maintain the coupling efficient for the light guide plate 12, a minimum distance L′ for coupling light generally has to be maintained between the light source 11 and the light guide plate 12, in a direction B′ of light coupling (i.e., a light entry direction of the light guide plate 12).
Referring to FIG. 1, in the single-long-side light entry type, the gravitational direction A′ of the light guide plate 12 and the light coupling direction B′ are parallel to each other. Even though the light guide plate 12 is caused to change (e.g., volume increased) due to external conditions, the change direction is along the gravitational direction A′ of the light guide plate 12. Therefore, this may make the light guide plate 12 closer to the light source 11. With the separation of the positioning posts 13, it can make sure that the minimum distance L′ for coupling light is maintained between the light source 11 and the light guide plate 12, thereby realizing the highest coupling efficiency and assuring that the light guide plate 12 would not touch the light source 11.
Referring to FIG. 2, however, in a single-short-side light entry type, the light source is disposed at the short edge 122 of the light guide plate 12. The gravitational direction A′ of the light guide plate and the light coupling direction B′ are perpendicular to each other. When the light guide plate 12 is caused to change due to external conditions, the change direction is still along the gravitational direction A′ of the light guide plate 12. At this moment, it may not make sure that the minimum distance L′ for coupling light is maintained between the light source 11 and the light guide plate 12. Also, this may further cause the light guide plate 12 to rotate, thereby causing coupling loss and decreasing brightness uniformity.
Above all, in the single-short-side light entry type, the gravitational direction of the light guide plate is perpendicular to the light coupling direction. When the light guide plate is caused to change along the gravitational direction, it may not ensure the minimum distance for coupling light between the light source and the light guide plate and this affects the coupling efficiency.